1. Field of the Invention
The present invention relates to a process for production of a heat-resistant hollow article made of a polylactic acid containing resin.
2. Description of the Related Art
In recent years, the use of food containers, such as bowls, cups and trays, which are made of biodegradable resin, has been investigated in restaurants, cafeterias and the like. Because the food containers which are made of biodegradable resin are degraded by the action of microorganisms in soil and the like after being discarded, it is possible to reduce the burden to the environment.
Hitherto, polylactic acid has been known as the above-described biodegradable resin. However, the polylactic acid has low heat resistance. Therefore, when the polylactic acid is used to make food containers, this poses the problems that the food containers cannot withstand hot water, heating by a microwave oven, washing and rinsing at temperatures around 70° C. in an alkali detergent for recycle after use, and high-temperature drying and sterilization treatment at temperatures around 80° C.
There are known biodegradable resin compositions that contain not less than 50% polylactic acid by mass and are nanocomposites of an inorganic filler as biodegradable resin compositions to solve the above-described problem (refer to Japanese Patent Laid-Open No. 2004-204143, for example). Excellent heat resistance can be imparted to the above-described biodegradable resin compositions by crystallization at temperatures near 110° C., and the above-described food containers can be formed by injection molding without being stretched.
In forming a hollow article such as the above-described food containers by injection-molding the biodegradable resin composition, for example, a mold that comprises a female mold portion having a concavity of a shape fitting the outer shape of the hollow article and a male mold portion having a convexity fitting the inner shape of the hollow article and opposed to the concavity, is heated to 110° C., for example, and the biodegradable resin composition in a molten state is injected into a cavity enclosed by the concavity and the convexity. And after the injected biodegradable resin composition is solidified to form the hollow article, the mold is opened to take out the hollow article.
However, it is likely that when an injection-molded product formed from the above-described biodegradable resin composition is taken out of the mold in an insufficiently cooled condition, it may be deformed because of insufficient rigidity and the shape of the injection-molded article may be deteriorated.
Also, an injection-molded product formed from the above-described biodegradable resin composition contracts if excessively cooled during solidification and tends to adhere to the above-described male mold portion, and during the opening of the mold, there may sometimes be a case where the injection-molded product cannot be easily released from the male mold portion, though it can be easily released from the female mold portion. Particularly, in the case where a hollow article such as a bowl or the like is formed through injection molding, it would be very difficult to release the injection-molded product from the mold if it is completely solidified.
Due to the above-mentioned problems, presently only articles such as a spoon, a fork and the like with low concavity or convexity can be produced through injection-molding from the biodegradable resin composition mentioned above. In order to solve the problem of the difficulty for releasing the mold caused by the strong contraction force, most of the polylactic acid-containing resins being currently distributed in the market are polymer-alloyed (for example, polylactic acid 40% and polycarbonate 60% by mass, or polylactic acid 50% and ABS 50% by mass, or the like). However, such polymer-alloyed polylactic acid containing resin contains a large amount of synthetic resin derived from petroleum oil and the synthetic resin is not biodegradable, and consequently causing a big problem to the environment.